1. Field of the Invention
The present invention relates generally to a differential amplifier with two outputs and a single input. Such an amplifier provides across the two output terminals a voltage proportional to the difference between a voltage received at the input terminal and a reference voltage. Such an amplifier is for example used in a digital versatile disk (DVD) reader.
2. Discussion of the Related Art
FIG. 1 schematically shows a conventional differential amplifier circuit with two outputs and a single input.
Amplifier 10 comprises an input terminal IN on which is applied a voltage VIN referenced to a reference potential GND, for example, the ground, and two output terminals OUT− and OUT+ respectively providing voltages VO− and VO+, also referenced to reference potential GND. Amplifier 10 comprises a first operational amplifier 12 comprising a non-inverting input (+) connected to terminal IN and an inverting input (−) connected to terminal S of amplifier 12. Amplifier 10 comprises a second operational amplifier 14 comprising an inverting input (−) connected to output S of the first amplifier 12 via a resistor 16 of value R1. The output and the inverting input (−) of operational amplifier 14 are connected via a resistor 18 of value R2. Output terminal OUT− of amplifier 10 corresponds to the output of operational amplifier 14. Operational amplifier 14 comprises a non-inverting input (+) connected to a terminal of a generator 20 of a reference voltage VREF having its other terminal connected to reference potential GND. Second output OUT+ of amplifier 10 is connected to output S of operational amplifier 12.
Since first operational amplifier 12 is connected as a follower, the voltage at output S, referenced with respect to reference potential GND, is equal to VIN. VO+ is thus equal to VIN. Since second amplifier 14 is connected as an inverter amplifier, VO− is equal to (1+R2/R1)VREF−(R2/R1)VO+.
Voltage VOUT between output terminals OUT+ and OUT− thus is in phase with VIN and has an amplitude substantially equal to the difference between VREF and VIN multiplied by an amplification factor 1+R2/R1. Such an amplifier thus provides a voltage VOUT having a peak-to-peak amplitude equal to the peak-to-peak amplitude of VIN multiplied by amplification factor 1+R2/R1. Further, since VOUT is obtained from the difference between VO+ and VO−, it is free of the noise present at the level of reference potential GND with respect to which VIN is referenced. Further, voltage VIN generally arrives onto the gate of the MOS transistor or the base of a bipolar transistor, according to the technology used to form operational amplifier 12. Amplifier 10 thus has a very high input impedance.
A disadvantage of such an amplifier is that it is not perfectly linear. Indeed, there generally exists a slight delay between VO− and VO+. Further, voltage VO− may be disturbed by specific noise, due to the resistances and to the different components of operational amplifier 14 which are present at the level of VOUT. Further, such an amplifier does not have a common mode control for both outputs OUT+ and OUT−. Finally, operational amplifier 14 generally introduces an additional phase and amplitude distortion of VO− with respect to VO+. As an example, the total harmonic distortion (THD) may be greater than 30 decibels for a maximum frequency smaller than 100 MHz. The linearity properties of such an amplifier may thus be insufficient for certain applications, for example, for a digital versatile disk (DVD) reader or for liquid crystal display control circuits.